UPDATE 31/5/22: Nintendo has now released a new patch for Nintendo Switch Online that removes this Kirby bug.
No patch notes have been released, but the N64 app is now at version 2.3.1. Finally, Kirby can swim once more.
ORIGINAL STORY 27/5/22: Following reports of a "game breaking" bug for the Switch Online version of Kirby 64: The Crystal Shards, Nintendo has confirmed a patch to fix the issue will arrive "early next week".
Kirby 64 launched as part of Nintendo's Switch Online + Expansion Pack retro collection last week, and players soon began reporting a bug in underwater levels - not seen in the 2000 original - that permanently stunned them after taking damage from certain sources.
In response to those reports, Nintendo has now confirmed a fix is on the way. "A bug has been found in Kirby 64: The Crystal Shards for Nintendo 64 - Nintendo Switch Online that can prevent players from progressing under a certain condition," it wrote in a tweet.
As a result of its findings, Nintendo says a patch to fix the issue will be release "early next week", adding, "We apologise for the inconvenience."
This isn't the first time Switch Online + Expansion Pack's N64 offerings have come in for criticism, of course; players highlighted input lag and control issues soon after the service's launch, alongside emulator peculiarities related specific games, but Nintendo has been working to address emulation concerns over time.